1. Field of the Invention
The present invention generally relates to a camera system and, more particularly, to a camera system which is constituted by a camera main body and a flash device between which the exchange of control signals, as data communication, is performed, and which is designed to decrease the number of signal lines between the camera main body and the flash device.
2. Description of the Related Art
When photography is to be performed by using a flash device, the device is either mounted (clipped) on a camera main body or is installed separately from the camera main body. When the flash device and the camera main body are placed at different positions, a cord is often used as a signal path for transmitting a control signal from the camera main body to the flash device so as to cause the flash device to produce flashlight.
If, however, a cord is used, a cumbersome wiring operation is required. In addition, as the user or the like tries to walk over the cord, he/she may catch his/her foot on the cord, thus damaging the camera main body or the flash device.
If, for example, emission control on the flash device is performed by radio using radio waves, infrared rays, visible light, or the like, no such inconvenience is caused.
If the flash device is placed at a position separated from the camera main body, every time a set value (emission mode, radiation angle, or the like) of the flash device is to be changed, the user must go to the installation position of the flash device to manually change the set value. This operation requires much time and labor. In order to avoid such a troublesome operation, the use of a signal path for data communication, in addition to a signal path for emission control, has been proposed. More specifically, when an operation of changing a set value of the flash device is performed on the camera main body side, the corresponding information is transmitted from the camera main body to the flash device through the signal path for data communication. The flash device then changes the set value on the basis of the information.
In order to realize this data communication, a camera system may be constructed, in which a radio signal path using electric waves, infrared rays, visible light, or the like is formed between a camera main body and a flash device, and proper modulation and demodulation are performed, like a system in which a plurality of computers communicate with each other through, e.g., telephone lines as signal paths, by modulating/demodulating signals using a modem.
A camera system having such an arrangement, however, requires at least two signal paths. In addition, such a camera system requires two circuits for demodulation and modulation, and hence an increase in cost is inevitable. In addition, an increase in the size of each device cannot be avoided.
Furthermore, in a camera system using signal paths such as radio stations for data communication, the number of usable signal paths (channels) is limited, besides channels are regarded as public resources. Therefore, it is undesirable to occupy two channels. In order to avoid this, communication for emission control and data communication may be performed through one signal path. However, communication for emission control must be performed in real time.
If an emission command and an emission stop command are communicated as data between the camera main body and the flash device, a time delay occurs. When, for example, an 8-bit signal as data is to be transmitted from the camera main body, since each transmission data requires a start bit and a stop bit, a period of time corresponding to at least 10 bits is required. Provided that the transmission rate is 1,200 bps, a time delay of at least 8.3 msec is caused. In such a camera system, therefore, the following problems are posed owing to this time delay: (i) an asynchronous curtain operation is caused unless the shutter speed is set to be sufficiently low, and (ii) an emission stop command cannot be transmitted, in effect, with a time delay of 8.3 msec, because the flash device produces flashlight within several msec upon reception of an emission command.